This invention relates to a drill hammer assembly and more particularly to a drill hammer assembly for a down the hole rock drill.
Down the hole rock drills are well known. Many drills of this kind have an air chamber above as well as below the piston. When the bottom chamber is pressurised with compressed air, the piston moves up and away from the bit. When the top chamber is pressurised the piston moves down towards the bit to strike the head of the bit. With each cycle, the chambers also have to exhaust. The bottom chamber does so by opening of the foot valve tube. The foot valve tube protrudes from the bit head here it engages in the bore of the piston. The exhaust air flows via the bore of the foot valve tube when it is open through the body of the bit and out to the atmosphere.
When the piston is in the upward stroke, there is a finger valve, which projects from the top chamber into the central bore of the piston to seal off the bore. When the piston moves towards the bit, the finger valve disengages from the bore of the piston and the top chamber exhausts through the central bore of the piston and then through the bore of the bit to the atmosphere.
All these constructions involve a reduction in the area of the piston exposed to working fluid due to the finger valve necessarily engaging in the piston bore.
It is advantageous to keep the overall diameter of the piston in a down-the-hole drill hammer assembly as small as possible while still operating an adequate piston area exposed to working fluid.
Accordingly it is the object of the present invention to provide a drill hammer assembly of the kind referred to in which reduction of piston area exposed to the working fluid by the finger valve is avoided.
According to this invention there is provided a down the hole drill hammer assembly comprising a backhead fitted into a wear sleeve, a piston forming the hammer reciprocable in the wear sleeve and a drill bit assembly including a foot valve tube and an anvil of smaller surface area than the hammer end of the piston located in the end of the wear sleeve remote from the backhead characterised in that the piston has blind bores extending from each end and ports through the wall of the piston providing for communication between the blind bores, a collar around the piston and the piston having a diameter at the hammer end larger than the backhead end, the backhead including an axial inlet passage and branched outlet passages through the wall of the backhead, longitudinal passages in the inner wall of the wear sleeve to provide selectively flow paths from the inlet to the upwardly exposed surfaces of the piston or alternatively to the upper surface of the collar and the drill bit end of the piston around the anvil with exhaust passages through the piston and communicating externally of the piston between the blind bores in the piston.
Further features of this invention provide a check valve located in the inlet passage.
The invention also provides for the bit assembly to be fitted in conventional manner to be axially movable in sealing engagement with the bottom end of the wear sleeve.